This invention relates to pattern tracers and, more particularly, to pattern tracers having kerf compensation features.
Pattern tracers of the type which photometrically detect and follow a pattern are well known. Examples of such tracers are shown in U.S. Pat. No. 4,160,199 of Bardwell; U.S. Pat. No. 4,128,297 of Bourne and U.S. Pat. No. 3,704,373 of Bardwell et al.
As discussed in these patents, the movement of a tracing element which follows the pattern is used to control the movement of a cutting tool along the surface of a work piece from which the pattern is being cut. If the pattern represents the precise dimensions of the piece to be cut, then compensation must be provided for kerf which is the width of the cut formed by the particular tool being employed.
There are two types of kerf: positive and negative. In those instances in which a hole is being cut from a work piece, the tracing element must follow the pattern laterally offset inwardly from the pattern at a distance equal to half the kerf. Compensation for such kerf is referred to as inner, or positive, kerf compensation. When a product is being cut from the interior of a work piece, then the tracing element must follow the pattern laterally offset outwardly from the pattern. Compensation for such kerf is referred to as outer, or negative, kerf compensation.
The magnitude of kerf is different for different tools and the sense of the kerf, either positive or negative, is different for different patterns. Accordingly, the kerf compensation must be adjustable with respect to both sense and magnitude.
Many known kerf adjustment techniques undesirably require mechanical adjustments which are awkward to use, subject to mechanical wear and unduly complicated. For instance, in Bardwell et al. U.S. Pat. No. 3,704,373, a pattern photodetector, a lens and other related elements are manually moved by means of a kerf adjustment knob. A mechanical adjustment technique is also apparently used in the system shown in Bourne U.S. Pat. No. 4,128,297.
Another undesirable feature of many known pattern tracers is the necessity of very precise alignment of the photo sensors used to detect the pattern. For instance, in a system using a circular array of sensors and a fixed coordinate system, a reference sensor must be in precise alignment with one of axis of the coordinate system. If not, all the angles detected by the photo detector will differ from the true pattern angles by an amount equal to the degree of misalignment and an associated erroneous offset will result in the tracing. Unfortunately, such mounting misalignment inherently occurs. In known pattern tracers, such misalignment is solved by remounting the sensors until they are in as close of an alignment as can be achieved.
Accurate cutting of the work piece under control of a pattern tracer also requires the pattern tracer to compensate for the thickness of the pattern line. In some instances it is desired to follow the center of the line, but in other instances, it is desired to trace alongside an inner or outer edge of a line. A line width compensation circuit for performing this function is shown in Bardwell U.S. Pat. No. 4,160,199. However, this circuit is limited to only a line width compensation function, and kerf compensation adjustment is provided by mechanical means.